From cb82bc80dacab75c06e936b7a7a359d49dbdb5a5 Mon Sep 17 00:00:00 2001
From: Jinwoo Hong <73622457+Jinwoo-H@users.noreply.github.com>
Date: Mon, 19 Oct 2026 03:33:15 -0400
Subject: [PATCH] fix(relay): never call a reserve cell's hosts stranded when
 it goes existing-only (#30609)

A reserve cell keeps no activity rows and its ledger stamps last_activity_at, so the
stranded rule read its connected hosts as unserved and re-placed them.
---
 .../assignment-reserve-mode-census.test.ts    | 21 +++++++++++++++++++
 cloud/apps/relay/src/assignment-store.ts      |  5 +++++
 2 files changed, 26 insertions(+)

diff --git a/cloud/apps/relay/src/assignment-reserve-mode-census.test.ts b/cloud/apps/relay/src/assignment-reserve-mode-census.test.ts
index a10b4994013..077552c8f03 100644
--- a/cloud/apps/relay/src/assignment-reserve-mode-census.test.ts
+++ b/cloud/apps/relay/src/assignment-reserve-mode-census.test.ts
@@ -228,6 +228,27 @@ describe('reserve-mode census', () => {
     await restore()
   })
 
+  it('keeps a connected host pinned when its reserve cell goes existing-only after a ledger write', async () => {
+    let now = 1_000
+    const store = await setup(() => now)
+    const identity = { userId: 'user-a', relayHostId: 'host000000000001' }
+    const first = await store.assign(identity)
+    await setReserve(store, [first.cellId])
+    // The reserve ledger stamps last_activity_at and the cell keeps no activity rows.
+    now += 5 * 60_000
+    await database!.query('DELETE FROM relay_assignment_activity_leases WHERE user_id = ?', [identity.userId])
+    await database!.query('UPDATE relay_assignments SET last_activity_at = ? WHERE user_id = ?', [now, identity.userId])
+    await store.setCellAdmissionState(first.cellId, 'existing-only')
+    now += 2 * 60_000
+    expect(await store.assign(identity)).toMatchObject({
+      cellId: first.cellId,
+      assignmentEpoch: first.assignmentEpoch
+    })
+    const { unknown, restore } = await unknownSet(() => now)
+    expect((await unknown.assign(identity)).assignmentEpoch).toBe(first.assignmentEpoch)
+    await restore()
+  })
+
   // Only the first reserve-set read is held; it is released (or failed) by the test.
   function holdFirstRead(options: { fail?: boolean } = {}) {
     const query = database!.query.bind(database!)
diff --git a/cloud/apps/relay/src/assignment-store.ts b/cloud/apps/relay/src/assignment-store.ts
index 66d90177bda..53dfcb5598f 100644
--- a/cloud/apps/relay/src/assignment-store.ts
+++ b/cloud/apps/relay/src/assignment-store.ts
@@ -1290,6 +1290,11 @@ export class RelayAssignmentStore {
     if (state !== 'existing-only') {
       return false
     }
+    // A reserve cell keeps no activity rows and its ledger stamps last_activity_at, so zero
+    // leases proves nothing; a dead one is left to the dead-reserve path and its fence.
+    if (await this.mayBeReserveCell(text(existing, 'cell_id'), transaction)) {
+      return false
+    }
     const liveLeases = (
       await transaction.query(
         `SELECT COUNT(*) AS live FROM relay_assignment_activity_leases